It is known to utilize check valves in tank-venting arrangements. In the following, a check valve which opens in response to overpressure in the tank will be referred to as an overpressure valve while a check valve which opens in response to underpressure will be referred to as an underpressure valve.
The simplest tank-venting arrangements include only an active charcoal filter between the tank and a tank-venting valve with the charcoal filter being provided with a venting line. An overpressure in the tank can be obtained to reduce the vapors from the fuel by arranging valves between the tank and the charcoal filter. A tank-venting arrangement of the kind described above is disclosed, for example, in U.S. Pat. No. 4,872,439. This tank-venting arrangement includes a check valve and an overpressure valve and an underpressure valve in parallel with the check valve and between the tank and the active charcoal filter. The check valve is opened when a tank closure is opened.
In various countries, it is required that the tank-venting arrangement be checked for tightness independently of whether a tank-venting arrangement is configured for normal pressure or overpressure. According to a suggestion of the California Environmental Authorities (CARB), the tightness is intended to be checked in that an investigation is made when the tank-venting valve is opened as to whether a lambda controller has to adjust its actuating signal in a "lean" direction. In this suggestion, the assumption is made that if the tank-venting arrangement is tight, then fuel vapor will have to be drawn out of the tank-venting arrangement by suction when the tank-venting valve is opened. If the air drawn in by suction by the internal combustion engine is enriched with fuel, it is then necessary to meter less fuel via the fuel-metering device in order to obtain a pregiven lambda value. For this purpose, the lambda controller has to change its actuating signal in the "lean" direction.
The problem however exists that cases often occur wherein hardly any fuel vapor is supplied from the tank-venting arrangement. This is especially the case when the fuel disposed in the tank has already lost all components having higher boiling points because of previous operation at higher temperature or when the fuel is relatively cool because of the ambient temperature or because of cooling by means of climate control equipment. In these cases, no lean correction is required by the lambda control when the tank-venting valve opens. The use of the suggested method would here cause a determination to be made that the tank-venting arrangement leaks even though no statement as to the degree of tightness of this arrangement is possible.